1. Field of the Invention
This invention relates to an apparatus for transmitting and receiving an amplitude modulated television signal in which a main ghost cancel reference signal is inserted and a specific signal in which a sub ghost canceling reference signal is inserted and which is multiplexed with the television signal.
2. Description of the Prior Art
More than 30 years have passed since the color television broadcasting of the current NTSC (National Television System Committee) system began in 1960. Recently, in search of a finer definition and higher performance television system, several new systems including the High Definition TV systems have been proposed. At the same time, the contents of the programs presented to viewers have been changed from the mere studio programs to programs providing higher quality images and more realistic feeling such as cinema-size movies.
The current NTSC broadcasting is standardized by 2:1 interlaced 525 scanning lines, luminance signal bandwidth of 4:2 MHz, and aspect ratio of 4:3. (See, for example, Pritchard, "US Color Television Fundamentals-A Review", IEEE Trans. Consumer Electron., vol. CE-23, pp. 467-478, November 1977.)
In this background, several television signal composition methods aiming at compatibility with the current broadcasting system, enhancement of horizontal resolution and extension of the aspect ratio have been proposed. One of such methods is presented in a paper by Yasumoto et al., "An extended definition television system using quadrature modulation of the video carrier with inverse Nyquist filter", IEEE Trans. Consumer Electron., vol. CE-33, pp. 173-180, August 1987. This paper describes a signal superposing method using quadrature modulation of the video carrier. By this method, various signals can be transmitted using a newly established quadrature channel with a very small interference to the conventional NTSC receiver in principle.
One of the big problems with the terrestrial broadcasting is multipath distortion of propagation waves. This problem has attracted much attention of TV viewers recently, because many tall buildings are built and reflect waves in the urban areas. When the quadrature channel is created using the above mentioned method, multipath distortion causes crosstalk between the main NTSC channel and multiplex signal.
In order to solve the multipath distortion, a Ghost Canceling Reference signal may be inserted during one of the vertical blanking lines of the main NTSC signal, as described in a paper by Miyazawa et al, "Development of a Ghost Cancel Reference signal for TV Broadcasting", IEEE Trans. on Broadcasting, Vol. 35, No. 4, December 1989, pp. 339-347. According to this paper, as shown in FIGS. 5 (a), (b) and FIG. 6, a sinx/x rising bar signal and a pedestal signal are inserted in the 18th and 281st horizontal lines by the 8-field sequence method. In FIG. 6, 17th line and 18th line are shown to clarify the advantage of the 8-field sequence GCR signal. In order to obtain the GCR signal from continuous 8 fields at the reception side, one can follow the equation: EQU GCR=1/4[(F0-F4)+(F5-F1)+(F2-F6)+(F7-F3)] (1)
where FX (X=0,1,2, . . . ,7) denotes Xth field signal.
If the image signal includes no motion in the 17th line (although normally there is no image signal in this line), this calculation cancels video signal, horizontal synchronous signal, and color sub-carrier, leading to get a pure GCR signal transmitted with a multipath distortion if any.
Quadrature modulation of the video carrier is one of the excellent techniques to keep the compatibility with the conventional TV broadcast. However, there arises crosstalk between two channels caused by a channel distortion and imperfectness of demodulation at the reception side. There is actually multipath distortion, or ghosts. In order to reduce multipath distortion with quadrature modulation, it is already proposed to insert Ghost Canceling Reference (GCR) signals into both of the NTSC and multiplex signals. For example, see a paper by Kageyama et al, "An NTSC Compatible Wide Screen Television System with Evolutionary Extendibility", IEEE Trans. on Consumer Electronics, Vol. CE-34, No. 3, August, 1988. According to this paper, as shown in FIG. 11, sinx/x GCR signals are inserted into horizontal lines during the vertical blanking period. In this case two horizontal lines are required for transmitting the GCR signals, one line for transmitting the main GCR signal and no signal for the multiplex channel, and the other for transmitting the multiplex GCR signal and black burst for the main channel. A disadvantage of this method is that this consumes two horizontal lines in each vertical blanking period.
FIG. 10 is a block diagram showing a 2-dimensional waveform equalizer suitable for equalizing the main NTSC signal and multiplex signal. This equalizer, consisting of four sets of series of multipliers, two sets of series of adders, and two series of delay lines, accepts and equalizes both the main NTSC signal and multiplex signal. Both signals generate tap weights, referring to themselves and counterpart, and compensate for ghosts caused by themselves and crosstalk from counterparts. All tap coefficients are provided by a control unit (not shown) after calculations referred to the GCR signals.